AS a model system for study of mechanisms of mammalian solid tumor and leukemia development and of host defenses against tumor growth, mast cell tumor transmission and growth in the beagle will be investigated. This highly malignant cancer is transplanted with nearly 100% assurance in puppies but is commonly rejected by older animals. The basis of tumor rejection in adult animals is unknown, but an immunological mechanism is likely. Since the disease can be transmitted by cell-free extracts, one aim of the work is to identify the causative agent or the active subcellular fraction. A second broad aim is to describe host mechanisms which modify the course of the disease. Mast tumor cells will be used as target cells for tests of cell-mediated (lymphocyte) and humoral (serum antibody) anti-tumor cytotoxic or enhancing factors which occur in response to tumor transplantation in the beagle. The immunological status of the animal will be manipulated through irradiation and use of immunosuppressive agents to permit appropriate studies in vivo. Anti-tumor effects of immune stimulation by specific and nonspecific immunization will be studied. A third aim is to study specific biochemical properties of these malignant cells, especially histamine, serotonin, and heparin metabolism, tumor-specific antigens, and the requirements for growth of the cells in culture in vitro. The work is expected to contribute to elucidation of immunobiochemical mechanisms of tumor growth and host defenses.